This invention relates generally to the subsurface nuclear density testing of coal stockpiles and more particularly to an improved method of performing nuclear density tests. In other aspect, the invention relates to a thin walled tubular housing for the nuclear depth density probe used in performing the tests.
In order for electrical utilities to maintain proper control of their coal supplies, it is necessary for the utilities to obtain accurate information as to the amount of coal that remains in their coal stockpiles. The stockpile quantity in tons is normally estimated by obtaining through testing an average density value of the stockpile and multiplying this average density by the measured volume of the stockpile. The density value is obtained by a testing technique known as subsurface nuclear density testing.
Nuclear density testing is carried out with an instrument known as a nuclear depth density gauge which measures the subsurface wet density of the coal stockpile by using backscatter absorption of nuclear radiation. The instrument includes a subsurface probe having a source of radiation and a sensing element or detector which receives the backscattered radiation. A recording meter known as a scaler remains at the surface and is connected with the detector by a five wire electrical cable. Measurements are taken by lowering the probe through an access hole to the desired depth in the stockpile. Measurements are usually taken at preselected depth intervals such as every two feet throughout the entire depth of the coal stockpile so that an accurate measurement of the average density is obtained.
The nuclear depth density instrument measures the density of a spherically shaped volume centered at the probe. Gamma radiation is emitted at a constant average rate from the radiation source in the probe, and the gamma rays interact at the atomic level with the surrounding medium. Since the number of scattering events per unit time is a function of the density of the medium, the backscatter measurement sensed by the detector provides a measure of the density of the stockpile. By sampling and analyzing a sufficient quantity of the backscattered radiation within a fixed energy range and on a per unit time basis, a statistically significant measure is obtained of the relative degree of scatter of the material at different densities. The analysis is in effect performed by the detector. The use of materials of known densities permits quantification of the various degrees of scatter in terms of density. Pulses from the radiation detector are applied through the five wire cable to the scaler instrument which displays the density information on a liquid crystal display or another type of display.
In the past, subsurface density measurements have been performed by two different methods, both of which carry out testing a depth intervals of between two and two and a half feet. One method is known as the split spoon method. In the split spoon procedure, a relatively large diameter bore hole (typically 6-8 inches) is initially drilled to the first test depth using hollow stem augers. A device known as a split spoon sampler is then screwed onto the end of the drill string and lowered through the inside of the hollow stem augers to the bottom of the bore hole. The split spoon sampler is essentially a thick wall pipe having an outside wall diameter of two inches and a length of two feet. After being lowered to the bottom of the bore hole by the drill string, the split spoon sampler is driven approximately two feet into the coal stockpile by a pin hammer mechanism. The drill string and sampler are then withdrawn from the bore hole, leaving a pilot hole at the bottom. The sampler is detached from the drill string, and a thin wall insert tube (0.035 inch wall thickness) is attached to the end of the drill rod and lowered through the hollow stem augers. The insert tube is hydraulically pushed into the pilot hole. After the drill string has again been withdrawn, the nuclear depth density probe is lowered through the hollow stem augers into the insert tube. Testing is carried out with the insert tube serving as protective casing for the pilot hole into which the probe is lowered.
After each test has been performed, the nuclear depth density probe is withdrawn, the bore hole is advanced to the next test depth by the hollow stem augers, the split spoon sampler is lowered to the bottom of the hole and driven into the coal to provide another pilot hole, the sampler is removed, the insert tube is lowered and pushed into the pilot hole, the probe is lowered into the insert tube, and the next test is performed. This procedure is repeated at 2-21/2 foot intervals until the entire depth of the coal stockpile has been tested.
Another and somewhat faster method of performing the density testing is referred to as the casing method. It involves first drilling an access hole through the entire depth of the coal stockpile using continuous flight augers. The access hole is cased by installing steel casing having an outside diameter of 2.25 inches (1/4 inches smaller in diameter than the access hole) and an inside diameter of 2 inches. After the access hole has been cased from top to bottom, the nuclear depth density probe is lowered into the casing and testing is performed at 2 or 21/2 foot intervals throughout the depth of the hole. When the testing has been completed, the probe is withdrawn, the casing is removed, and the process is repeated at the next test location.
As can easily be appreciated, the casing method is much less cumbersome and much quicker than the split spoon method, primarily because the split spoon method requires repeated lowering and raising of the drill string to manipulate the split spoon sampler and install and replace the insert tube. It has been found that the casing method takes on average approximately half as long as the split spoon method. During an eight hour working day, approximately 30 to 35 tests can be performed with the split spoon method, as contrasted with 60 to 70 tests with the casing method. This advantage of the casing method in speed is particularly significant in view of the high cost of labor and the high cost of operating drilling equipment.
However, the split spoon method provides much greater accuracy than the casing method. The greater accuracy is attributable to the thin wall of the insert tube (0.035 inch) in comparison to the relatively thich wall (0.125 inch) of the casing through which the radiation must pass in the casing method of testing. Since the amount of radiation which is able to enter the surrounding medium is greatly reduced with increasing wall thickness, the sensitivity of the nuclear depth density instrument is decreased when a thick wall must be penetrated. The variations in the density of the casing (such as where the casing sections are joined) also detract from the accuracy of the casing method, along with the unsymmetrical nature of the generally annular gap formed between the casing and the bore hole. Accordingly, the accuracy that is obtained from the thin wall split spoon method gives it a significant advantage over the thick wall casing method of testing. The need to use hollow stem augers in the split spoon procedure is a disadvantage which is substantially offset by the need to case the entire depth of the access hole in the casing method.